A global influenza pandemic is imminent and will kill up to 150 million people, the UN official in charge of coordinating the worldwide response to an outbreak has warned.

David Nabarro, one of the most senior public health experts at the World Health Organisation, said outbreaks of bird flu, which have killed at least 65 people in Asia, could mutate into a form transmittable between people.

"The consequences in terms of human life when the pandemic does start are going to be extraordinary and very damaging," he said.

He told the BBC that the "range of deaths could be anything between five and 150 million".

A highly pathogenic form of bird flu, known as the H5N1, has led to the culling of tens of millions of birds in south-east Asia, but efforts to contain it have not prevented it spreading as far as the Ural mountains in Russia.

Carried by wildfowl, it spreads quickly among poultry flocks and has killed people living or working in close proximity to infected birds. Scientists fear the virus could evolve into a form which could be passed from human to human with catastrophic consequences.

Last month Neil Ferguson, a professor of mathematical biology at Imperial College London, told Guardian Unlimited that up to 200 million people could be killed.

"Around 40 million people died in 1918 Spanish flu outbreak," said Prof Ferguson. "There are six times more people on the planet now so you could scale it up to around 200 million people probably."

A Department of Health contingency plan states anywhere that there could be between 21,500 and 709,000 deaths in Britain.

Last week, veterinary and medical chiefs from the European Union held talks aimed at drawing up an EU-wide action plan to prevent the spread of bird flu. Experts say spotting any outbreak immediately and treating local people with anti-viral drugs and vaccines will be the key to containing any outbreak.

Rich countries are stockpiling anti-viral supplies. Britain announced in March that it was spending £200m on treatments for up to 14 million people. In July the government also said it would buy 2m doses of vaccine for key workers, though it will take around six months for it to arrive.

The problem facing governments and the WHO is that it is difficult to know what vaccine to produce until an outbreak occurs and then to manufacture treatments in sufficient quantities.

"A flu outbreak is imminent but no one knows if it will be next week or in three years' time," a WHO spokeswoman said. "It is really difficult to know how many people will be infected but we know we have to get prepared."

She said the "best case scenario" would be 7.4 million deaths globally.

South-east Asia's agriculture ministers announced a regional plan today to combat bird flu and pledged to co-operate with international agencies in stamping out the virus.

The ministers from the 10-member Association of South-east Asian Nations (ASEAN) said in a statement that bird flu requires "an all-out coordinated regional effort".